Abstract Adolescents in the juvenile justice system demonstrate very high rates of tobacco, alcohol, and other drug use (ATOD), with rates that are estimated to be three times higher than non-justice-involved youth. Substance- abusing youth ? especially those who are involved in the juvenile justice system ? are at higher risk than nonusers for mental health problems, including depression, conduct problems, personality disorders, suicidal thoughts, attempted suicide, and completed suicide. Additionally, there is evidence of detrimental effects on brain development and stress responsive neurobiological systems from substance abuse. There is growing evidence that interventions based on mindfulness have been an efficacious intervention for a variety of problems, including substance use among adolescents. We propose to adapt and test the feasibility and efficacy of a smart phone application (app) intervention prototype that would help adolescent substance users reduce or quit their substance use. The program, entitled Rewire, is based on the primary substance use cessation components tested in our previous work with juvenile justice-involved adolescents and on intervention components shown to be central to smoking cessation, and applies a mindfulness approach as the guiding framework for the intervention. The twelve module program, which includes 24 components (12 teaching components and 12 practice components) for each module, will be evaluated with 380 high risk adolescents that are involved in the juvenile justice system. The proposed program will be the only mindfulness-based mobile app specifically targeting the reduction or cessation of adolescent substance use; there is high level of need for the program both as an adjunct to existing treatments and for use in the juvenile justice system. Our research and development team has extensive experience in working with adolescents that use ATOD, and we have been using mindfulness as the framework for in-person interventions with youth in the juvenile justice system as well as through the Phase I development of the Rewire app. The research team will partner with Twenty Ideas, who has extensive experience in the development of mobile apps for smartphones, and who was the software development company for Phase I. We will assess navigation and usability with high risk adolescents who are currently reporting substance use. Additional pilot testing will be used to guide development of the Rewire app for evaluation. The evaluation study will involve having 380 youth ages 13-18 involved in the juvenile justice system use the app for 1 month and complete baseline, 1- and 3-month follow- up. Usability data, program engagement, changes in attitudes, emotions and behaviors, and self-reported substance use will be used to determine the efficacy of the Rewire mobile app.